Aluminium-magnesium alloys resistant to blackening and elements formed thereof



United States Patent Int. (:1. 6m 21/02 U.S. Cl. 75-147 2 Claims ABSTRACT OF THE DISCLOSURE An aluminum-magnesium alloy which can be prepared of unrefined aluminum and which does not blacken in the presence of non-deionized water, said alloy having magnesium present as an alloying element in an amount within the range of 3% to 5% by weight and iron and silicon present as impurities with the weight ratio of iron to magnesium being not more than 0.04 and the weight ratio of iron plus silicon to magnesium being not more than 0.07, and other products formed thereof.

This invention relates to aluminum-magnesium alloys and more particularly to an alloy of the type described which is not blackened by non-deionized water even when formed of unrefined aluminum. The invention also relates to the utilization of alloys of the type described in the manufacture of aluminum products and particularly culinary utensils.

In the present context, the expression refined aluminum refers to aluminum which is at least 99.99% pure and in which the impurities are iron, silicon and copper (see LAluminium, Edition Eyrolles 1964, Part I, page 256).

Generally speaking, unrefined aluminum and its alloys have a tendency to blacken at the surface when in contact with water unless the water is completely deionized.

The extent to which blackening occurs depends somewhat upon the period of contact, the type of alloy and the type and concentration of ions present in the water. Blackening occurs more rapidly with increase in the temperature of the water. Although the phenomenon of blackening is superficial and harmless, it nevertheless has a detrimental effect on the appearance of the objects produced, particularly when in the form of kitchen utensils or food containers.

Although there has been extensive research on this subject, a satisfactory solution to the problem has not yet been achieved. In an article published in Zeit Metallkunde, Fischer and Geller reported that refined aluminum, having a purity of more than 99.99% and its alloys with 0.5% to 1% magnesium or with 1% zinc do not turn black on contact with ordinary boiling water. However, it is uneconomical to make use of such highly refined aluminum in the manufacture of domestic utensils.

In a German patent of P. Brenner, No. 1,109,381, the suggestion is made that certain heat treatments at elevated temperature Will be effective to prevent the blackening of aluminum-magnesium, aluminum-zinc or aluminum-manganese alloys. Unfortunately, actual experience does not confirm the results disclosed in this patent. Although temporary improvement is secured, blackening occurs when the metal is in prolonged contact with boiling water.

It is an object of this invention to produce a new and improved aluminum-magnesium alloy formed of unrefined aluminum, which does not blacken in response to exposure to non-deionized water and it is an object to produce new and improved utensils employing same.

In accordance with the practice of this invention, formulation is made of an alloy based on unrefined aluminum and magnesium in which the magnesium is present in an amount within the range of 3% to 5% and in which the iron content is such that the weight ratio of iron to magnesium is equal to or less than 0.04 while the weight ratio of iron silicon magnesium is equal to or less than 0.07.

The invention also has for its object the manufacture of components and particularly kitchen utensils obtained from this alloy which do not turn black on contact with nondeionized water.

The theoretical concepts behind the phenomenon of blackening have always been somewhat obscure and research to find a satisfactory solution to the problem has remained empirical. Through the work of the inventors, it has been possible to establish that the discoloration is related to the polarity of the heterogeneities in the metal in relation to its mass. These heterogeneities in the metal consist of intermediate elements which may emanate from the constituents of the alloy or from the impurities which inevitably accompany unrefined aluminum.

The heterogeneities which show a positive electric potential in relation to the mass of metal seem to be responsible for the blackening of the surface of alumina. Those which are more electro-negative than the metal do not cause any discoloration. Examples of electropositive heterogeneities include manganese, copper and iron. Magnesium and combinations of silicon and magnesium are examples of electronegative heterogeneities.

It is impractical to use refined aluminum for the production of domestic utensils because of its great expense. Instead, a less pure aluminum is employed which also contains small proportions of iron and silicon. Applicants have found that it is possible to neutralize the harmful effects of iron by adding magnesium in an amount to adjust the weight ratio or iron to magnesium to 0.04 or less, while the weight ratio of iron silicon magnesium is 0.07 or less.

On the other hand, if the alloy is to remain readily workable and non-corrodable, it should contain magnesium in an amount within the range of 3% to 5% by weight. This then fixes the maximum iron content of the aluminum capable of being employed in the production of the alloy.

99.7% to 99.9% pure aluminum, which is directly produced by electrolysis, identified in France as A7 to A9 type alloy, is suitable for use in the production of alloys of this invention because the iron content is within the range of 0.1% to 0.2%.

The alloy of this invention is produced by the usual method and is subjected to conventional mechanical and heat treatments to produce sheeting from which the end products can be fabricated.

The following examples are given by way of illustration, but not by way of limitation, of the practice of this invention:

EXAMPLE 1 An aluminum alloy is produced from an aluminum ingot of the A7 type in which the principal impurities are 0.17% iron and 0.09% silicon. Magnesium is added in an amount to constitute 5% of the aluminum alloy by melting the two metals after which it is cast, rolled into sheets and annealed by the conventional methods. The

sheet so obtained is stamped into a saucepan. The test to determine resistance to blackening is carried out under carefully controlled conditions in order to minimize errors in interpretation due simply to a delay in the start of blackening.

An artificial sea water formulated of an aqueous solution of 3% sodium chloride, 0.125% boric acid and 0.018% disodium phosphate was used in the test. The saucepan was half filled with the test water which was then kept boiling for 6 hours during which evaporation was avoided by means of a cooled lid. The metal showed no difiference in color between the part immersed as compared to the part above the water line.

EXAMPLE 2 An alloy was prepared in the same manner as in Example 1 with magnesium present in an amount of 3% instead of 5% and tested under identical conditions. The immersed portion blackened. The Fe/Mg ratio of 0.056 was too high to prevent blackening.

EXAMPLE 3 An alloy containing 3% magnesium was prepared from aluminum A85 having an iron content of 0.12% and a silicon content of 0.09%. The procedure of Example 1 was followed. There was no visible difference in color between the portion immersed as compared to the portion above the water line.

These examples demonstrate that it is possible to make use of unrefined aluminum in the production of aluminum-magnesium alloys which are resistant to blackening in the presence of water and even in the presence of strongly ionized water provided that the iron content, present as an impurity, is present in a weight ratio of not more than 0.04 relative to magnesium.

It will be apparent from the foregoing that we have provided a new and improved alloy system of aluminum .4 and magnesium wherein an alloy resistant to blackening can be produced from unrefined aluminum which is of lower cost and sufiiciently readily available to enable use in the fabrication of utensils and other products.

It will be understood that changes may be made in the details of construction, arrangement and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. Culinary utensils which do not blacken in the presence of non-deionized water and formed of an alloy consisting essentially of magnesium present as an alloying element in an amount within the range of 3% to 5% by weight, iron and silicon being present, the weight ratio of iron to magnesium being limited to not more than 0.04 and the weight ratio of iron +silicon magnesium being limited to not more than 0.07, and the balance unrefined aluminum.

2. A culinary utensil as claimed in claim 1 in which the alloy makes use of unrefined aluminum of 99.7% to 99.9% by weight purity.

References Cited UNITED STATES PATENTS 1,935,608 11/1933 Archer 147 2,058,429 10/1936 Edwards 75-147 2,079,786 5/1937 Beck 75-147 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 75142 

